The is write-up describes two simple fixes for the SPF GT40 speedo cable and speedo readout error. As delivered, the SPF GT using the RBT ZF trans will read about 82 MPH at an actual speed of 60 MPH. This is because the drive gear that comes with the ZF is wrong for this installation. The obvious solution is to get the proper gear from RBT to solve the problem right? FORGET IT !!!! All one has to do is search on this forum about trying to get anything from RBT and you will understand my comment above. Dennis Olthoff at Olthoff Racing has had adapters made that mate up to the SPF/ZF configuration and it takes care of the problem beautifully. Dennis sells these adapters for about $100.
A couple notes are in order however. When installing the adapter, don't over tighten the nut on the trans, just bottom it out and then back it off ¼ turn. On the cable end, the thread pitch is very tight so apply a dab of anti-seize compound to the threads and again bottom out the nut and back it off ¼ turn. If you over tighten these nuts, it causes the cable to bind and potentiality break. It also causes the speedo needle to jump and give erratic readings.
The next problem to tackle is at the speedo end. My cable broke after about 600 miles. I know of several others who have had similar experiences. I'm sorry for the horrible photo below, but what it shows is the arc that the cable made because of the way the cable is jammed up against the fuse/relay box.
As you can see below, the cable shaft broke two inches from the end.
You need to acquire a 90 degree adapter that mounts between the cable and the speedo. Find one with 5/8-18 threads, no gear reduction and 104 square drive. Stewart Warner part number 407010 is perfect but can be difficult to find. There are suitable subs also and they are available from Fleet Parts & instrument Co. 1-800-582-3912 for about $75. Just ask for the SW number when you order.
It's a tight fit but it works and your cable will last a great deal longer.
A couple notes are in order however. When installing the adapter, don't over tighten the nut on the trans, just bottom it out and then back it off ¼ turn. On the cable end, the thread pitch is very tight so apply a dab of anti-seize compound to the threads and again bottom out the nut and back it off ¼ turn. If you over tighten these nuts, it causes the cable to bind and potentiality break. It also causes the speedo needle to jump and give erratic readings.
The next problem to tackle is at the speedo end. My cable broke after about 600 miles. I know of several others who have had similar experiences. I'm sorry for the horrible photo below, but what it shows is the arc that the cable made because of the way the cable is jammed up against the fuse/relay box.
As you can see below, the cable shaft broke two inches from the end.
You need to acquire a 90 degree adapter that mounts between the cable and the speedo. Find one with 5/8-18 threads, no gear reduction and 104 square drive. Stewart Warner part number 407010 is perfect but can be difficult to find. There are suitable subs also and they are available from Fleet Parts & instrument Co. 1-800-582-3912 for about $75. Just ask for the SW number when you order.
It's a tight fit but it works and your cable will last a great deal longer.
